Motion vector detection apparatus, motion vector detection method, and computer-readable storage medium

ABSTRACT

A motion vector detection apparatus comprising, a memory for storing a reference image for motion prediction encoding, a unit for detecting a first motion vector by comparing the reference image with an encoding target block of a plurality of blocks obtained by dividing a field image, a converter for converting the field image into a frame image by performing interlace/progressive conversion, a unit for generating a reduced image by reducing the frame image, a frame memory for storing the reduced image, and a unit for detecting a second motion vector based on a reference reduced image and a reduced image of the encoding target block of reduced images stored in the frame memory, wherein the second motion vector detected with respect to the reduced image of the encoding target block is used to determine a search area for detecting the first motion vector.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a motion vector detection apparatus, amotion vector detection method, and a computer-readable storage medium.

2. Description of the Related Art

In recent years, digitization of information associated with so-calledmultimedia such as audio signals and video signals is rapidly becomingcommon. Along with this, compression encoding/decoding techniques forvideo signals are receiving attention. Compression encoding/decodingtechniques can reduce memory capacity necessary for storing videosignals and bandwidth necessary for transmission, and is thereforeextremely important for the multimedia industry.

Compression encoding/decoding techniques compress the information/dataamount using high autocorrelation (that is, redundancy) present in mostvideo signals. Redundancies of video signals include time and spatialredundancies. It is possible to decrease time redundancy using motiondetection and compensation for each block. Furthermore, it is possibleto decrease spatial redundancy using discrete cosine transformation(DCT).

In the MPEG scheme known as a compression encoding/decoding technique orthe like, such methods decrease the redundancy of video signals, therebyimproving the data compression effect of a video frame/field whichvaries with time. Motion estimation for each block for decreasing timeredundancy is a task for finding a block most resembling that in acurrent frame/field among blocks in sequentially input referenceframes/fields (previous frames/fields). A vector representing a movingdirection and moving amount of a corresponding block is called motionvector. The motion detection is synonymous with motion vector detection.In such motion vector detection, a video signal is divided into blockseach consisting of m pixels×n lines (m and n are integers) and servingas a unit of motion vector detection (e.g., a macro block), and a motionvector is detected for each macro block. In motion vector detection, itis possible to use a block matching method disclosed in Japanese PatentLaid-Open No. 2004-229150, and the like.

SUMMARY OF THE INVENTION

To encode an interlaced video signal, motion detection is performedbetween fields. In this case, when motion vector detection is performedfor an object having an oblique line with small motion as an edge, it isdifficult to accurately recognize the motion. That is, a motion vectormay be erroneously detected, resulting in video quality degradationafter encoding.

The present invention provides a motion compensation encoding techniquein which even if motion between field images is small when encoding aninterlaced video signal, a motion vector is not erroneously detected andtherefore video quality degradation hardly occurs.

According to one aspect of embodiments, the present invention relates toa motion vector detection apparatus comprising, a memory configured tostore a reference image for motion prediction encoding; a motion vectorsearch unit configured to detect a first motion vector by comparing thereference image stored in the memory with an encoding target block of aplurality of blocks obtained by dividing a field image, a converterconfigured to convert the field image into a frame image by performinginterlace/progressive conversion, a reduced image generation unitconfigured to generate a reduced image by reducing the frame image, aframe memory configured to store the reduced image, and a pre-motionvector search unit configured to detect a second motion vector based ona reference reduced image and a reduced image of the encoding targetblock of reduced images stored in the frame memory, wherein the motionvector search unit uses the second motion vector detected with respectto the reduced image of the encoding target block to determine a searcharea for detecting the first motion vector with respect to the encodingtarget block divided from the field image.

Further features of the present invention will become apparent from thefollowing description of exemplary embodiments (with reference to theattached drawings).

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram showing an example of the arrangement of amotion compensation encoding apparatus according to the firstembodiment;

FIGS. 2A and 2B are views for explaining a problem when encoding aninterlaced video signal;

FIG. 3 is a view for explaining an interlace/progressive conversionscheme;

FIG. 4 is a block diagram showing an example of the arrangement of amotion compensation encoding apparatus according to the secondembodiment; and

FIG. 5 is a flowchart illustrating motion vector detection processingaccording to the first embodiment.

DESCRIPTION OF THE EMBODIMENTS

A detailed description will be given based on embodiments of the presentinvention with reference to the accompanying drawings.

[First Embodiment]

With reference to FIGS. 1 and 5, an embodiment of a compensationencoding apparatus including a motion vector detection apparatusaccording to the present invention will be explained in detail. FIG. 1is a block diagram showing a motion compensation encoding apparatusaccording to the present invention. FIG. 5 is a flowchart illustratingan example of motion vector detection processing according to theembodiment.

This embodiment is characterized by a technique for decreasing erroneousdetection of a motion vector (the first motion vector) for performingmotion compensation on an encoding target block (e.g., macro block) in afield image. More specifically, a field image is converted into a frameimage to generate a reduced image, a pre-motion vector (the secondmotion vector) is detected with respect to the reduced image, and thesecond motion vector is used to search for the first motion vector.

The reason why erroneous detection may occur in motion vector detectionbased on a field image will be described next with reference to obliquelines in still images shown in FIGS. 2A and 2B. As shown in FIG. 2A,when encoding a progressive video signal, video is processed as frames.In this case, the frames have the same pattern in the time direction,and oblique lines are present in the identical coordinates in theframes, thereby correctly detecting a motion vector to be “0”. On thecontrary, when encoding an interlaced video signal, video is processedas fields. In this case, as shown in FIG. 2B, patterns in even-numberedfields and odd-numbered fields are different from each other, andtherefore, oblique lines are not present in identical coordinates acrossframes. This causes a detection error because the motion vector does notbecome “0”. Consequently, when playing back encoded video, visual videoquality degradation (image quality degradation) occurs.

Processing in the embodiment will be explained in detail using eachcomponent shown in FIG. 1. Each block of a motion compensation encodingapparatus 100 in FIG. 1 may be configured by a hardware component usinga dedicated logic circuit or memory. Alternatively, each block may beconfigured by a software component when a CPU executes a processingprogram stored in a memory. The flowchart in FIG. 5 can be implementedwhen the CPU, functioning as a corresponding functional block, executesthe corresponding program (stored in a ROM or the like).

In step S501, an interlace/progressive converter 101 converts aninterlaced video signal into a progressive video signal, and thentransmits it to a reduced image generation unit 102.

An example of the interlace/progressive conversion processing will bedescribed with reference to FIG. 3. Note that any method of generating aprogressive video signal is applicable. FIG. 3 shows a case in whichidentical coordinate positions are extracted for each pixel from each oftwo sequential field images (interlaced video signals). At this time,the coordinate positions are extracted by considering an interpolationtarget pixel (interpolation point) 301 of a current field as a center.To perform interlace/progressive conversion, whether there was motion ofthe interpolation pixel is determined using values at upper and lowerpositions of the interpolation point 301 of the current field, and avalue at the same position as that of the interpolation point 301 of thecurrent field in a previous field immediately before the current field.Note that b and c respectively represent the values at the upper andlower positions of the interpolation point 301 of the current field, anda represents the value at the same position as that of the interpolationpoint 301 of the current field in the previous field, as shown in FIG.3. A difference value fdd between the fields is calculated using thosevalues according to:fdd=|a−(b+c)/2|  (1)

As fdd increases, the difference between the current and previous fieldsalso increases, thereby determining that there was a motion. On thecontrary, if fdd becomes small, the difference between the current andprevious fields also becomes small, thereby indicating that there was nomotion. For example, fdd is compared with a predetermined value(threshold TH1). If fdd>TH1, it is possible to determine that “there wasmotion”; otherwise, it is possible to determine that “there was nomotion”.

If it is determined that “there was no motion”, the interpolation pointis considered to have the same value as that in the previous field, andthe value (“a” in FIG. 3) in the previous field is substituted intact.This interpolation processing method is called “inter-fieldinterpolation”. If it is determined in the motion determinationprocessing that “there was motion”, the interpolation point has a valueindependent of the value in the previous field. Therefore, an averagevalue ((b+c)/2 in FIG. 3) of the upper and lower pixel values in thecurrent field is set to an interpolation value. This interpolationprocessing method is called “intra-field interpolation”.

In step S502, the reduced image generation unit 102 generates a reducedimage. As a method of generating a reduced image, a method of using anaverage value of pixel values of two pixels successively arranged in thevertical direction and four pixels successively arranged in thehorizontal direction when reducing an image to ½ in the verticaldirection and ¼ in horizontal direction may be used but any other methodis also applicable. Note that in this embodiment, a case in which animage is reduced to ½ in the vertical direction and ¼ in the horizontaldirection will be described as an example.

A pre-motion vector search reference frame memory 103 stores reducedimages of progressive video sent from the reduced image generation unit102 in a display order, and sequentially transmits encoding targetblocks to a pre-motion vector search unit 104 in an encoding order. Thememory 103 also stores the reduced images of the progressive video asreference reduced images for searching for pre-motion vectors, andsequentially transmits them to the pre-motion vector search unit 104.Note that since pre-motion vector search is performed for a reducedimage of progressive video, it is performed by adjusting the size of anencoding target block to the size of a reduced image.

In this embodiment, since the progressive video signal is reduced to ½in the vertical direction and ¼ in the horizontal direction, the videosignal is increased twofold in the vertical direction, and then a blockreduced to ½ in the vertical direction and ¼ in the horizontaldirection, that is, a block reduced to ¼ in the horizontal direction isused. For example, since the size of an encoding target block in MPEG2is 16×16 pixels, pre-motion vector search is performed using a blockwith a size of 4×16 pixels.

The pre-motion vector search unit 104 searches for a pre-motion vectorin step S503. In this case, vector detection is performed based on theposition at which correlation is strong between an encoding target macroblock from the pre-motion vector search reference frame memory 103 and areference image from the pre-motion vector search reference frame memory103.

To estimate a motion vector having the strongest correlation, anevaluation function such as MSE (Mean Square Error) (equation 2), MAE(Mean Absolute Error) (equation 3), or MAD (Mean Absolute Difference) isused.

$\begin{matrix}{{M\; S\;{E( {i,j,k} )}} = {\frac{1}{UV}{\sum\limits_{u = 0}^{U}{\sum\limits_{v = 0}^{V}\lbrack {{S_{{cur},k}( {x,y} )} - {S_{ref}( {{x + i},{y + j}} )}} \rbrack}}}} & (2) \\{{M\; A\;{E( {i,j,k} )}} = {\frac{1}{UV}{\sum\limits_{u = 0}^{U}{\sum\limits_{v = 0}^{V}{{{S_{{cur},k}( {x,y} )} - {S_{ref}( {{x + i},{y + j}} )}}}}}}} & (3)\end{matrix}$where S_(ref) represents a reference image, and S_(cur,k) represents thekth macro block in the current frame. Furthermore, (i, j) indicates thespatial position of a reference image for the kth macro block in thecurrent frame.

Assume that X represents the number of pixels arranged in the horizontaldirection in a search window and Y represents the number of pixelsarranged in the vertical direction in the search window. In this case,x=g×u and y=h×v where g and h are natural numbers satisfying 0≦x≦X,1≦g≦X, 0≦y≦Y, and 1≦h≦Y. Moreover, U and V satisfy X−g≦U≦X and Y−h≦V≦Y.

The evaluation functions are based on the difference between pixels. Avector having the smallest MAE value or MSE value is selected as apre-motion vector in a current macro block. Note that since pre-motionvector search is performed using a reduced image of progressive video,the size of the reduced image needs to be the same as the image size ofthe interlaced video. In this embodiment, a detected pre-motion vectoris increased fourfold in the horizontal direction. Thereafter, thedetermined pre-motion vector is transmitted to a motion vector searchunit 105 and a post-filter reference frame memory 118.

The motion vector search unit 105 searches for a motion vector in stepS504. Note that the unit 105 searches for a vector using the pre-motionvector from the pre-motion vector search unit 104 based on a position atwhich correlation between an encoding target macro block and a referenceimage from the post-filter reference frame memory 118 becomes strong.More specifically, the unit 105 may narrow, based on the pre-motionvector, an area of the reference image to undergo motion vector searchprocessing, or may acquire a reference image shifted by the pre-motionvector from the post-filter reference frame memory 118. Alternatively,the unit 105 may perform both the above processes to acquire a referenceimage shifted by the pre-motion vector, and then further narrow a searcharea within the reference image.

To estimate a motion vector having the strongest correlation, anevaluation function such as MSE or MAD described above is used, like apre-motion vector. A vector having a smallest MAE value or MSE value isselected as a motion vector in a current macro block. Note that sincemotion search is performed using interlaced video intact, the size of ablock and the magnitude of a motion vector need not be adjusted, unlikepre-motion vector search.

Information about the determined motion vector is sent to a motioncompensation unit 106 and an entropy encoder 112. At the same time,reference image identification information used for generating themotion information is also sent to the motion compensation unit 106 andthe entropy encoder 112.

As described above, the interlace/progressive converter 101, reducedimage generation unit 102, pre-motion vector search reference framememory 103, and pre-motion vector search unit 104 are used to detect thepre-motion vector as the second motion vector. Then, the motion vectorsearch unit 105 and post-filter reference frame memory 118 can use thepre-motion vector to detect a motion vector as the first motion vector.These components can constitute the motion vector detection apparatusaccording to this embodiment.

An intra prediction unit 107 divides the reconstructed image data in apre-filter reference frame memory 116 into blocks of a predeterminedsize, and predicts reconstructed image data within each block based onthe values of the pixels surrounding the block. The unit 107 calculatesthe predicted value as predicted image information, and sends it to aswitch 108. The switch 108 is switched depending on a prediction methodby a controller (not shown). In the case of an intra prediction encodingscheme, the switch is connected to a position 108 a, and data obtainedby a calculation method based on the intra prediction method is sent aspredicted image information.

The motion compensation unit 106 reads out a reconstructed image for thereference image corresponding to the reference frame identificationinformation from the post-filter reference frame memory 118, andgenerates predicted image information of a current image based on thereconstructed image data and the motion vector. As described above, aninter prediction encoding scheme is different from the intra predictionencoding scheme in that predicted image information is generated withreference to a frame different from the current image.

Predicted image information generated in the inter prediction encodingscheme is connected to a position 108 b using the switch 108, and dataobtained by a calculation method based on the inter prediction method issent as predicted image information. A subtracter 109 subtracts, from anencoding target block, a predicted image block transmitted from theswitch 108, and outputs image residual data. An orthogonaltransformation unit 110 performs orthogonal transformation processingfor the image residual data output from the subtracter 109, and thentransmits a transformation coefficient to a quantization unit 111.

The quantization unit 111 uses a predetermined quantization parameter toquantize the transformation coefficient transmitted from the orthogonaltransformation unit 110, and transmits the quantized transformationcoefficient to the entropy encoder 112 and an inverse quantization unit113. The entropy encoder 112 inputs the transformation coefficientquantized by the quantization unit 111, performs entropy encoding suchas CAVLC or CABAC, and then outputs the result as encoded data.

Next, a method for generating a reference image using the transformationcoefficient quantized by the quantization unit 111 will be described.The inverse quantization unit 113 inverse quantizes the quantizedtransformation coefficient transmitted from the quantization unit 111.An inverse orthogonal transformation unit 114 inverse orthogonaltransforms the transformation coefficient inverse quantized by theinverse quantization unit 113 to generate decoded residual data, andtransmits it to an adder 115. The adder 115 adds the decoded residualdata to predicted image data (to be described later) to generate areference image, and stores it in a pre-filter reference frame memory116. The adder 115 also transmits the reference image to a loop filter117. Note that the reconstructed image data at this time has lowerquality than that of the input image data due to a predicted informationerror or a quantization error in the quantization processing.

The loop filter 117 performs predetermined filtering processing forpixel data adjacent to the boundaries of a block, thereby suppressingdiscontinuity of data at the boundaries of the block. As describedabove, the reconstructed image data has lower quality than that of theinput image. In image data processed for each block in each process,discontinuity of data is likely to occur at the boundaries of the block,and this is recognized as block noise. To reduce block noise, adeblocking filter is used. The reconstructed image data that hasundergone the boundary processing is stored in the post-filter referenceframe memory 118.

As described above, even when encoding interlaced video, by usingprogressive video in upper layer motion vector search in layered motionvector search, it is possible to decrease erroneous detection of amotion vector even in video whose motion is small.

[Second Embodiment]

With reference to a block diagram in FIG. 4, another embodiment of amotion vector detection apparatus according to the present inventionwill be described in detail. A motion vector detection apparatus of amotion compensation encoding apparatus 100 according to the secondembodiment shown in FIG. 4 has almost the same arrangement as in thefirst embodiment but also has an encoding target macro block motiondetermination unit 119 and pre-motion vector storage unit 120. Thisembodiment is different from the first embodiment in that a motiondetermination result of an interlace/progressive converter 101 istransmitted to the encoding target macro block motion determination unit119, and a motion vector search method of a pre-motion vector searchunit 104 is switched depending on an encoding target macro block motiondetermination result.

The operations of components except for the interlace/progressiveconverter 101, pre-motion vector search unit 104, encoding target macroblock motion determination unit 119, and pre-motion vector storage unit120 are the same as in the first embodiment, and a description thereofwill be omitted. The interlace/progressive converter 101 determinesinterlace/progressive conversion target pixel motion using equation 1used in the first embodiment, and transmits a determination result tothe encoding target macro block motion determination unit 119. Theencoding target macro block motion determination unit 119 receives themotion determination result from the interlace/progressive converter101, and determines the encoding target macro block motion, and thentransmits the determination result to the pre-motion vector search unit104.

An encoding target macro block motion determination method will beexplained below. The encoding target macro block motion determinationunit 119 receives the motion determination result from theinterlace/progressive converter 101, and counts the number of pixels inthe encoding target macro block, for which it is determined that therewas a motion. Assume that the count is indicated by MV_(cnt). Whetherthe encoding target block moved is determined usingMV_(cnt)≧TH2   (4-1)MV_(cnt)<TH2   (4-2)where TH2 represents a predetermined threshold. If the inequality 4-1holds, the number of moved pixels is determined to be large, and it istherefore determined that the encoding target macro block moved. On theother hand, if inequality 4-2 holds, the number of moved pixels isdetermined to be small, and it is therefore determined that the encodingtarget macro block did not move.

The pre-motion vector search unit 104 changes a search positiondepending on the encoding target macro block motion determination resulttransmitted from the encoding target macro block motion determinationunit 119 to search for a motion vector. If it is determined that theencoding target macro block did not move, the unit 104 determines asearch area based on the position of the encoding target macro block,and requests a reference image of a pre-motion vector search referenceframe memory 103. The unit 104 then uses the obtained reference image toperform a motion search around the encoding target macro block.

Alternatively, if it has been determined that there was a motion, theunit 104 determines the search area based on the position of theencoding target macro block and a pre-motion vector (to be referred toas a “prediction motion vector”) obtained in the surrounding macroblock. The unit 104 requests a reference image of the pre-motion vectorsearch reference frame memory 103. Note that the surrounding macro blockincludes, for example, a macro block which has undergone pre-motionvector detection, and is adjacent to the encoding target macro block. Ifthere are a plurality of targets surrounding macro blocks, the resultobtained by combining a plurality of pre-motion vectors can be used as aprediction motion vector. As described above, the pre-motion vectorsearch unit 104 shifts a search position by the prediction motion vectorto perform motion search. As in the first embodiment, MSE, MAE, or MADis obtained. A vector with the smallest MAE value or MSE value isselected as a pre-motion vector in a current macro block.

Since pre-motion vector search is performed using a reduced image ofprogressive video, it is necessary to adjust the size of the reducedimage to the image size of interlaced video. In this embodiment, adetected pre-motion vector is enlarged fourfold in the horizontaldirection. The determined pre-motion vector is then transmitted to amotion vector search unit 105 and a post-filter reference frame memory118. The pre-motion vector is also transmitted to the pre-motion vectorstorage unit 120 within the pre-motion vector search unit. Subsequentencoding processing is the same as in the first embodiment, and adescription thereof will be omitted.

In this embodiment, a position where movement is expected is specifiedusing a pre-motion vector obtained in a surrounding macro block.However, other methods may also be used. For example, instead of theencoding the target macro block motion determination unit 119 and thepre-motion vector storage unit 120, there may be provided a globalvector search unit which indicates a spatial difference (that is, ashift amount between fields) of an encoding target image with respect toa reference image. The global vector search unit detects a globalvector, and outputs it to the pre-motion vector search unit 104. Thepre-motion vector search unit 104 may search for a pre-motion vector ata position shifted from an encoding target macro block by the globalvector.

To detect a global vector, it is possible to use an evaluation functionsuch as MSE (Mean Square Error), MAE (Mean Absolute Error), or MAD (MeanAbsolute Difference) like a motion vector. Examples of the evaluationfunction using MSE and MAE are given by

$\begin{matrix}{{M\; S\;{E( {i,j} )}} = {\frac{1}{QR}{\sum\limits_{q = 0}^{Q}{\sum\limits_{r = 0}^{R}\lbrack {{S_{cur}( {{m + i},{n + j}} )} - {S_{ref}( {m,n} )}} \rbrack^{2}}}}} & (5) \\{{M\; A\;{E( {i,j} )}} = {\frac{1}{QR}{\sum\limits_{q = 0}^{Q}{\sum\limits_{r = 0}^{R}{{{S_{cur}( {{m + i},{n + j}} )} - {S_{ref}( {m,n} )}}}}}}} & (6)\end{matrix}$where S_(cur)(m, n) represents the (m, n)th pixel value in a currentframe, and S_(ref)(m, n) represents the (m, n)th pixel value in areference image. Furthermore, (i, j) indicates a spatial position of thecurrent frame with respect to the reference image.

Assume that M represents the number of pixels arranged in the horizontaldirection in one frame, and N represents the number of pixels arrangedin the vertical direction in one frame. In this case, m=k×q and n=l×rwhere k and l are natural numbers satisfying 0≦m≦M, 1≦k≦M, 0≦n≦N, and1≦l≦N. Furthermore, Q and R satisfy M−k≦Q≦M and N−l≦R≦N.

As described above, in this embodiment, if an encoding target block doesnot move, pre-motion vector search is performed around an encodingtarget macro block; otherwise, pre-motion vector search is performed ata position where it is expected that there is motion. This makes itpossible to decrease the probability of erroneously detecting a motionvector.

Other Embodiments

Aspects of the present invention can also be realized by a computer of asystem or apparatus (or devices such as a CPU or MPU) that reads out andexecutes a program recorded on a memory device to perform the functionsof the above-described embodiment(s), and by a method, the steps ofwhich are performed by a computer of a system or apparatus by, forexample, reading out and executing a program recorded on a memory deviceto perform the functions of the above-described embodiment(s). For thispurpose, the program is provided to the computer for example via anetwork or from a recording medium of various types serving as thememory device (for example, computer-readable medium).

While the present invention has been described with reference toexemplary embodiments, it is to be understood that the invention is notlimited to the disclosed exemplary embodiments. The scope of thefollowing claims is to be accorded the broadest interpretation so as toencompass all such modifications and equivalent structures andfunctions.

This application claims the benefit of Japanese Patent Application No2010-172726, filed Jul. 30, 2010, which is hereby incorporated byreference herein in its entirety.

What is claimed is:
 1. An apparatus comprising: a converter thatgenerates a progressive image from a first field image and a secondfield image prior to the first field image by using a conversion method,wherein the conversion method includes (a) determining a differencebetween an average value of pixels at both sides of a first pixel in thefirst field image and a pixel value of a second pixel which is in thesecond field image and corresponds to the first pixel, (b) generatingthe first pixel based on the average value to generates the progressiveimage if the difference exceeds a predetermined value, and (c)generating the first pixel based on the second pixel to generates theprogressive image if the difference does not exceed the predeterminedvalue; a reduced image generating unit that generates a reduced imagefrom the progressive image; a second motion vector detecting unit thatdetects a second motion vector based on the reduced image; and a firstmotion vector detecting unit that detects, from a search area, a firstmotion vector corresponding to a target block included in the firstfield image, wherein the search area is determined based on the secondmotion vector.
 2. The apparatus according to claim 1, wherein the firstmotion vector detecting unit determines the search area by narrowingdown, based on the second motion vector, an area for searching the firstmotion vector.
 3. The apparatus according to claim 1, wherein the firstmotion vector detecting unit determines the search area by shifting,based on the second motion vector, an area for searching the firstmotion vector.
 4. A non-transitory computer readable storage medium thatstores a program for causing a computer to execute a method, the methodcomprising: generating a progressive image from a first field image anda second field image prior to the first field image by using aconversion method, wherein the conversion method includes (a)determining a difference between an average value of pixels at bothsides of a first pixel in the first field image and a pixel value of asecond pixel which is in the second field image and corresponds to thefirst pixel, (b) generating the first pixel based on the average valueto generates the progressive image if the difference exceeds apredetermined value, and (c) generating the first pixel based on thesecond pixel to generates the progressive image if the difference doesnot exceed the predetermined value; generating a reduced image from theprogressive image; detecting a second motion vector based on the reducedimage; and detecting, from a search area, a first motion vectorcorresponding to a target block included in the first field image,wherein the search area is determined based on the second motion vector.5. A method comprising: generating a progressive image from a firstfield image and a second field image prior to the first field image byusing a conversion method, wherein the conversion method includes (a)determining a difference between an average value of pixels at bothsides of a first pixel in the first field image and a pixel value of asecond pixel which is in the second field image and corresponds to thefirst pixel, (b) generating the first pixel based on the average valueto generates the progressive image if the difference exceeds apredetermined value, and (c) generating the first pixel based on thesecond pixel to generates the progressive image if the difference doesnot exceed the predetermined value; generating a reduced image from theprogressive image; detecting a second motion vector based on the reducedimage; and detecting, from a search area, a first motion vectorcorresponding to a target block included in the first field image,wherein the search area is determined based on the second motion vector.6. An apparatus comprising: a converter that generates a progressiveimage from a first field image and a second field image prior to thefirst field image by using a conversion method, wherein the conversionmethod includes (a) determining a difference between an average value ofpixels at both sides of a first pixel in the first field image and apixel value of a second pixel which is in the second field image andcorresponds to the first pixel, (b) generating the first pixel based onthe average value to generates the progressive image if the differenceexceeds a predetermined value, and (c) generating the first pixel basedon the second pixel to generates the progressive image if the differencedoes not exceed the predetermined value; a second motion vectordetecting unit that detects a second motion vector based on theprogressive image; and a first motion vector detecting unit thatdetects, from a search area, a first motion vector corresponding to atarget block included in the first field image, wherein the search areais determined based on the second motion vector.
 7. The apparatusaccording to claim 6, wherein the first motion vector detecting unitdetermines the search area by narrowing down, based on the second motionvector, an area for searching the first motion vector.
 8. The apparatusaccording to claim 6, wherein the first motion vector detecting unitdetermines the search area by shifting, based on the second motionvector, an area for searching the first motion vector.
 9. Anon-transitory computer readable storage medium that stores a programfor causing a computer to execute a method, the method comprising:generating a progressive image from a first field image and a secondfield image prior to the first field image by using a conversion method,wherein the conversion method includes (a) determining a differencebetween an average value of pixels at both sides of a first pixel in thefirst field image and a pixel value of a second pixel which is in thesecond field image and corresponds to the first pixel, (b) generatingthe first pixel based on the average value to generates the progressiveimage if the difference exceeds a predetermined value, and (c)generating the first pixel based on the second pixel to generates theprogressive image if the difference does not exceed the predeterminedvalue; detecting a second motion vector based on the progressive image;and detecting, from a search area, a first motion vector correspondingto a target block included in the first field image, wherein the searcharea is determined based on the second motion vector.
 10. A methodcomprising: generating a progressive image from a first field image anda second field image prior to the first field image by using aconversion method, wherein the conversion method includes (a)determining a difference between an average value of pixels at bothsides of a first pixel in the first field image and a pixel value of asecond pixel which is in the second field image and corresponds to thefirst pixel, (b) generating the first pixel based on the average valueto generates the progressive image if the difference exceeds apredetermined value, and (c) generating the first pixel based on thesecond pixel to generates the progressive image if the difference doesnot exceed the predetermined value; detecting a second motion vectorbased on the progressive image; and detecting, from a search area, afirst motion vector corresponding to a target block included in thefirst field image, wherein the search area is determined based on thesecond motion vector.